There are plenty of roads in Idaho that scientists could have used to test out how highways and byways impact wildlife, but researchers in Boise decided to make their own—a phantom road.

In 2012, Boise State University researchers started studying how road noise could be harming migratory songbirds near Idaho’s Lucky Peak State Park. But there was no road there at all: It just sounded like one.

They recorded road noise from highways in Glacier National Park—one of the nation’s most heavily visited parks—and piped the noise through speakers set up near the university’s bird-banding location at Lucky Peak State Park.

Road noise has long been thought to change birds’ behavior, but Heidi Ware, education director at Boise State University’s Intermountain Bird Observatory, and fellow postdoctoral researcher Christopher McClure wanted to filter out other factors—such as light pollution, air pollution from vehicles, and collisions—to nail down how much influence noise really has on animal behavior.

“When you’re trying to find out what impacts road noise can have on wildlife, it’s hard to tease out the other side effects that come with a road,” said Ware.

Jesse Barber hanging phantom road speakers. (Photo: Chris McClure)

For two years, the team monitored how 51 different migratory songbird species handled noise at the phantom road site compared with birds that landed about a mile away and out of range of the sound. They also captured, tagged, and took measurements of different birds at the two sites, so they could see how long they stayed put and whether they gained the weight needed to continue on their migration.

The team’s work has been published in a paper released this week in the Proceedings of the National Academy of Sciences. The study found that road noise directly affects birds’ foraging abilities, lengthens stopover times during migration, and limits weight gain—all signs that noise pollution is an “invisible source of habitat degradation,” the scientists wrote.

“This is an issue, because birds need stopover sites like Lucky Peak to replenish their fuel tank so they can make the trip across regions that lack the types of nutrition they can get here,” Ware said.

Many of these migrating birds come from Canada, and they head as far south as South America. Birds typically use Lucky Peak as a one-week stopover point, before heading over the Idaho’s 70-mile-wide Snake River Plain, which is dry, infested with predators, and lacking food.

It’s a tough journey. Pipe in road noise, and it gets worse. Compared with the ambient noise location, about one-third fewer birds stayed to forage at the phantom road site, and those that did decide to stay ended up foraging for a longer period of time. That’s because their foraging habits changed, Ware said.

In a video clip, the researchers demonstrated how a white-crowned sparrow forages with and without road noise. When it’s quiet, the bird focuses on the food, hopping around in search of berries. But when the speakers are turned on, the sparrow’s head is up, showing a “higher vigilance rate,” according to the study. It’s covering less ground and eating less food. That means birds that stay in the noisy zones will either be malnourished during their flight or will stay behind a few days longer to bulk up. Both are bad options, Ware said.

Bird species like MacGillivray’s warblers and western tanagers were substantially lighter if they foraged near the phantom road site, and they ended up staying around longer too.

“If they don’t get enough food, they’ll be less likely to make the migration, and if they stay too long, they’re going to get to the wintering grounds last, after the other birds have already staked their claim to the best territory,” Ware said.

Possible solutions include new, quieter road materials to lessen tire noise, more shuttle operations in national parks to limit traffic, and more electric cars.

But while electric cars put out fewer decibels compared with gasoline-powered vehicles, a federal rule in the works could make electric cars of the future a problem.

“They want to make them louder, because people often can’t hear them coming, and that could be bad for wildlife,” Ware said.

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